In fact, the impedance control routine is 10% deviation. A slightly stricter one can achieve 8%. There are many reasons:

1. The deviation of the sheet material itself

2. Etching deviation in PCB processing

3. Deviations such as flow rate caused by lamination during PCB processing

4. At high speed, the roughness of the surface of the copper foil, the glass fiber effect of PP, the DF frequency change effect of the medium, etc.

To understand impedance, you must understand processing. In the next few articles, let’s take a look at some knowledge of processing. The first one will look at lamination:

1. The principle of PCB pressing

The main purpose of lamination is to combine PP with different inner core boards and outer copper foils through “heat and pressure”, and use the outer copper foil as the base of the outer circuit. And different PP composition with different inner plate and surface copper can be equipped with different specifications and thickness of circuit boards. The pressing process is the most important process in the manufacture of PCB multilayer boards, and it must meet the basic quality indicators of PCB after pressing.

1. Thickness: Provides related electrical insulation, impedance control, and glue filling between inner layers.

2. Combination: Provide bonding with inner black (brown) and outer copper foil.

3. Dimensional stability: The dimensional change of each inner layer is consistent to ensure the alignment of the holes and rings of each layer.

4. Board warping: Maintain the flatness of the board.

2. PCB pressing process

The conditions that must be met for the pressing process

A. Material conditions:

The inner core board of the conductor pattern is made

Copper foil

Prepreg

B. Process conditions:

high temperature

high pressure

3. Introduction to PP of laminated material

characteristic:

Properties of prepreg

A. RC% (Resin content): refers to the weight percentage of the resin component in the film except for the glass cloth. The amount of RC% directly affects the resin’s ability to fill the gaps between the wires, and at the same time determines the thickness of the dielectric layer after pressing the board.

B. RF% (Resin flow): refers to the percentage of the resin flowing out of the board to the total weight of the original prepreg after pressing the board. RF% is an index reflecting the fluidity of the resin, and it also determines the thickness of the dielectric layer after pressing the plate

C. VC% (volatile content): refers to the percentage of the original weight of the volatile components lost after the prepreg is dried. The amount of VC% directly affects the quality after pressing.

Function:

1. As the bonding medium of the inner and outer layers.

2. Provide an appropriate insulating layer thickness. The film is composed of glass fiber cloth and resin. The thickness difference of the same glass fiber cloth film after pressing is mainly adjusted by the different resin content rather than the pressing conditions.

3. Impedance control. Among the main four influencing factors, the Dk value and the thickness of the dielectric layer are determined by the characteristics of the film. The Dk value of the formed film can be roughly calculated by the following formula.

Dk=6.01-3.34R R: Resin content%

Therefore, the Dk value used when estimating the impedance can be calculated based on the ratio of the glass fiber cloth and the resin in the laminated film combination.

The actual thickness of PP after filling is calculated as follows:

Thickness after PP pressing

1. Thickness = theoretical thickness of single PP-filling loss

2. Filling loss = (1-A surface inner layer copper foil residual copper rate) x inner layer copper foil thickness + (1-B surface inner layer copper foil residual copper rate) x inner layer copper foil thickness/3, inner layer Residual copper rate = inner wiring area / entire board area

The residual copper rates of the two inner layers in the above figure are as follows:

Please pay attention to the above formula. If we are calculating the filling loss of the secondary outer layer, we only need to calculate one side, not the residual copper rate of the outer layer. as follows:

Filling loss = (1-inner copper foil residual copper rate) x inner copper foil thickness

Compression structure design

(1) Thin core with larger thickness is preferred (relatively better dimensional stability)

(2) The low-cost PP is preferred (for the same glass cloth type PP, the resin content basically does not affect the price)

(3) The symmetrical structure is preferred to avoid PCB warpage after the finished product. The following figure is a non-scale structure and is not recommended.

(4) The thickness of the dielectric layer》the thickness of the inner copper foil×2

(5) It is forbidden to use PP with low resin content in a single sheet between 1-2 layers and n-1/n layers, such as 7628×1 (n is the number of layers)

(6) For 3 or more prepregs arranged together or the thickness of the dielectric layer is greater than 25 mils, except for the outermost and innermost layers of PP, the middle PP is replaced by a light board

(7) When the second layer and n-1 layer are 2oz bottom copper and the thickness of the 1-2 and n-1/n layers of insulating layer is less than 14mil, it is forbidden to use single PP, and the outermost layer needs to use high resin content PP. Such as 2116, 1080; if the residual copper rate is less than 80%, try to avoid using a single 1080PP

(8) The inner layer of copper 1oz board, when 1-2 layer and n-1/n layer use 1 PP, the PP needs to use high resin content, except 7628×1

(9) It is forbidden to use single PP for boards with inner copper ≥ 3oz. Generally, 7628 is not used. Multiple PPs with high resin content must be used, such as 106, 1080, 2116…

(10) For multilayer boards with copper-free areas greater than 3″×3″ or 1″×5″, PP is generally not used as a single sheet between the core boards.